The present invention relates to a protective device for electric motors, especially stepper motors. In particular it relates to such protective devices which have means for determination of operational temperature of an electric motor and means for automatic switching off of the electric motor in the event if a predetermined limiting operational temperature is exceeded.
In electric motors high operational temperatures are produced in the event of high and/or long loading during the operation or for example when the electric motor is blocked and nevertheless is connected to a network or a battery. High operational temperatures can lead to an increased wear of carbon brushes, commutators and bearings and in the extreme case can lead to winding short-circuiting by damaging of the isolation.
For preventing unpermissibly high operational temperatures, it is known to provide such electric motors with thermoprotective switches formed for example as a bimetal switch arranged in a circuit of the electric motor. When the excessive temperature is exceeded, the bimetal contact opens and the electric motor is switched off. Such a bimetal contact is however not only place consuming and expensive, but also very inaccurate. Moreover, it is often very difficult to arrange it at the locations which are critical with respect to a temperature increase.